Firestore 获取 DocumentSnapshot 字段的值

Question

如果我有一个 Firebase Firestore 数据库，我检索了与右侧集合对应的文档的DocumentSnapshot并存储在document变量中，那么我如何检索该DocumentSnapshot中“用户名”字段的值？ 该字段有一个字符串值。

Answer 1

DocumentSnapshot有一个getString()方法，它接受字段的名称并将其值作为字符串返回。

String value = document.getString("username");

Answer 2

您可以使用get方法获取字段的值

String username = (String) document.get("username");  //if the field is String
Boolean b = (Boolean) document.get("isPublic");       //if the field is Boolean
Integer i = (Integer) document.get("age")             //if the field is Integer

查看DocumentSnapshot

Answer 3

您需要执行DocumentReference才能获取DocumentReference中的内容。

一个简单的会是这样的。

DocumentReference docRef = myDB.collection("users").document("username");
docRef.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
@Override
     public void onComplete(@NonNull Task<DocumentSnapshot> task) {
          if (task.isSuccessful()) {
               DocumentSnapshot document = task.getResult();
                    if (document != null) {
                         Log.i("LOGGER","First "+document.getString("first"));
                         Log.i("LOGGER","Last "+document.getString("last"));
                         Log.i("LOGGER","Born "+document.getString("born"));
                    } else {
                         Log.d("LOGGER", "No such document");
                    }
               } else {
                    Log.d("LOGGER", "get failed with ", task.getException());
                }
          }
     });

缺点是您需要知道您的文档 ID 才能获取字段值。

Answer 4

当我在 onComplete 内部时，我只能将字段的数据作为字符串引用，但是当我尝试在它外部引用它时。 我得到一个 nullPointerException 并且它使我的活动崩溃。

// Gets user document from Firestore as reference
    DocumentReference docRef = mFirestore.collection("users").document(userID);

    docRef.get().addOnCompleteListener(new OnCompleteListener<DocumentSnapshot>() {
        @Override
        public void onComplete(@NonNull Task<DocumentSnapshot> task) {
            if (task.isSuccessful()) {
                DocumentSnapshot document = task.getResult();
                if (document.exists()) {

                    Log.d(TAG, "DocumentSnapshot data: " + document.getData());
                    Log.d(TAG, "db firstName getString() is: " + document.getString("firstName"));
                    Log.d(TAG, "db lastName getString() is: " + document.getString("lastName"));

                    mFirstName = (String) document.getString("firstName");
                    mLastName = (String) document.getString("lastName");
                    Log.d(TAG, "String mFirstName is: " + mFirstName);
                    Log.d(TAG, "String mLastName is: " + mLastName);

                } else {
                    Log.d(TAG, "No such document");
                }
            } else {
                Log.d(TAG, "get failed with ", task.getException());
            }
        }
    });

    //string checking outside the docRef.get().addOnCompleteListener code
    //commented it out because it causes a java.lang.NullPointerException: println needs a message
    //Log.v("NAME", mFirstName);
    //Log.v("NAME", mLastName);

    // sets the text on the TextViews
    tvFirstName = (TextView)findViewById(R.id.tvFirstName);
    tvFirstName.setText(mFirstName);
    tvLastName = (TextView)findViewById(R.id.tvLastName);
    tvLastName.setText(mLastName);

Answer 5

这是获取文档值的另一种简单方法（在您的情况下）：

Firestore.instance
.collection('users').document('xsajAansjdna')
.get()
.then((value) =>
print("Fetched ==>>>"+value.data["username"]));

Answer 6

`Firestore.instance
.collection('users').document('xsajAansjdna')
.get()
.then((value) =>
print("Fetched ==>>>"+value.data()["username"]));`